For guidance and/or steering purposes, all manned and unmanned mobile platforms, such as land vehicles, powered airborne platforms such as aircrafts and rockets, and non-powered airborne platforms such as gun-fired munitions and mortars, require onboard information as to their absolute position and orientation (usually relative to the earth) or their position and orientation relative to another object such as a reference platform or a target object.
In certain cases, the onboard position and certain orientation information (absolute or relative to the target, a reference station, another mobile platform, etc.) can be provided by an outside source, for example, by GPS for position or by a radar reading or optical signal that is reflected off some target or received by the mobile platform. In other cases, it is either required or is highly desirable to have autonomous sensors on board the mobile platform, including gun-fired projectiles, mortars and missiles, to directly measure the position and orientation of the object with respect to earth or some fixed object (for example a ground station) or a moving object (for example a moving target).
It is noted that even though in this disclosure all references are made to moving platforms, it will be appreciated by those of ordinary skill in the art that the provided description also includes the measurement of the position and orientation of one object relative to another object, one or both of which may be fixed to a third object such as the ground.
Currently available sensors that could make partial or full measurement of the position and/or orientation of an object relative to earth or another object (reference system) can be generally divided into the following classes of sensors.
One class of position and angular orientation sensors operates using optical methods. Such sensory systems can directly measure angular position of one object relative to another. However, optical based angular position sensory systems suffer from several disadvantages, including operation only in the line of sight between the two objects; accurate measurement of relative angular orientation only if the objects are relatively close to each other; limited range of angular orientation measurement; relatively high power requirement for operation; requirement of relatively clean environment to operate; and in military applications the possibility of exposing the site to enemy and jamming. Optical gyros do not have most of the above shortcomings but are relatively large, require a considerable amount of power, and are difficult to harden for high G firing accelerations. Optical methods such as tracking of projectiles with surface mounted reflectors and the like have also been developed, which are extremely cumbersome to use even during verification testing, suffer from all the aforementioned shortcomings, and are impractical for fielded munitions. In addition, the information about the object orientation can usually be determined only at the ground station and has to be transmitted to the moving object for guidance and control purposes. As a result, optical angular position sensors are generally not suitable for munitions and other similar applications.
Another class of angular orientation sensors is magnetometers that can be used to measure orientation relative to the magnetic field of the earth. The main problem with magnetometers is that they cannot measure orientation of the object about the magnetic field of the earth. Other important issues are low sensitivity; requirement of an accurate map of the magnetic field in the area of operation; and sensitivity to the presence of vehicles and the like in the area, the configuration of which usually varies in time, particularly in an active war theatre.
Another class of position and angular orientation measurement systems is based on the use of radio frequency (RF) antennas printed or placed on the surface of an object to reflect RF energy emanating from a ground-based radar system. The reflected energy is then used to track the object on the way to its destination. With two moving objects, the radar measures the time difference between the return signals from each of the objects and thereby determines angular information in terms of the angle that the relative velocity vector makes with respect to a coordinate system fixed to one of the objects. With such systems, measurement of full spatial orientation of an object (relative to the fixed radar or a second object) is very difficult. In addition, the information about the object orientation is determined at the radar station and has to be transmitted back to the moving object(s) if it is to be used for course correction. It is also very difficult and costly to develop systems that could track multiple projectiles. It is noted that numerous variations of the above methods and devices have been devised with all suffering from similar shortcomings.
In addition to the above angular orientation measurement sensors, GPS is often used to provide position information in the horizontal plane (i.e., orthogonal to the direction of gravity) and direction of the object travel. The GPS, however, does not provide altitude and angular orientation information. In the particular case of munitions, the use of GPS alone has a number of significant shortcomings, particularly for munitions applications in general and gun fired munitions, mortars and rockets in particular. These include the fact that GPS signals may not be available along the full path of the flight, and the measurements cannot be made updated fast enough to make them suitable for guidance and control purposes.
Another class of position and angular orientation sensors is based on utilizing polarized Radio Frequency (RF) reference sources and mechanical cavities as described in U.S. Pat. Nos. 6,724,341 and 7,193,556 and 7,425,998 and U.S. patent application Ser. No. 12/189,183 filed on Aug. 10, 2008, now abandoned, the entire disclosures of each of which are incorporated herein by reference, and hereinafter are referred to as “polarized RF angular orientation sensors”. These angular orientation sensors use highly directional mechanical cavities that are very sensitive to the orientation of the sensor relative to the reference source due to the cross-polarization and due to the geometry of the cavity. The reference source may be fixed on the ground or may be another mobile platform (object). Being based on RF carrier signals, the sensors provide a longer range of operation. The sensors can also work in and out of line of sight. In addition, the sensors make angular orientation measurements directly and would therefore not accumulate measurement error. The sensor waveguides receive and record the electromagnetic energy emitted by one or more polarized RF sources. The angular position of a waveguide relative to the reference source is indicated by the energy level that it receives. A system equipped with multiple such waveguides can then be used to form a full spatial orientation sensor. In addition, by providing more than one reference source, full spatial position of the munitions can also be measured onboard the munitions. These angular orientation sensors are autonomous, i.e., they do not acquire sensory information through communication with a ground, airborne or the like source. The sensors are relatively small and can be readily embedded into the structure of most mobile platforms including munitions without affecting their structural integrity. As a result, such sensors are inherently shock, vibration and high G acceleration hardened. A considerable volume is thereby saved for use for other gear and added payload. In addition, the sensors become capable of withstanding environmental conditions such as moisture, water, heat and the like, even the harsh environment experienced by munitions during firing. In addition, the sensors require a minimal amount of onboard power to operate.
Currently available sensors for remote measurement of the angular orientation of an object relative to the earth or another object (target or weapon platform) rely mostly on inertia-based sensors. This class of sensors measure changes in the angular position using inertial devices such as accelerometers and gyros. Inertial based angular orientation sensors, however, generally suffer from drift and noise error accumulation problems. In such sensors, the drift and the measurement errors are accumulated over time since the acceleration has to be integrated to determine the angular position. As a result, the error in the angular position measurement increases over time. In addition, the initial angular orientation and angular velocity of the object must be known accurately. Another shortcoming of inertia based angular position sensors is that the position and angular orientation of one object relative to another cannot be measured directly, i.e., the orientation of each object relative to the inertia frame has to be measured separately and used to determine their relative angular orientation. As a result, errors in both measurements are included in the relative angular orientation measurement, thereby increasing the error even further. In addition, electrical energy has to be spent during the entire time to continuously make such sensory information.
In the particular case of gun-fired munitions, to achieve the required guidance and control accuracy over relatively long distances and related times, the position and angular orientation of the projectile has to be known during the entire time of the flight with high precision. The firing acceleration, however, would saturate the inertial devices and require relatively long periods of time to settle. As a result, such sensors need to be initialized often, particularly for their initial position and orientation following firing and settling of the inertial devices. For longer range weapons and to further increase precision, the inertial devices may also have to be initialized regularly during the flight.
For initialization of position in the horizontal plane and heading (direction of path of travel of the projectile—which would generally indicate the mean angular orientation of the projectile in the horizontal plane for stable flights), the GPS may be used when available or when necessary for guidance purposes towards a target. However, other means have to be used to determine the altitude and angular orientation in the vertical plane, and in particular the roll angle of the projectile, i.e., its angular orientation about its long axis (which is usually close but not always coincident to the tangent to path of travel of the center of mass of the projectile).
The elevation and projectile orientation in the vertical plane is important if the projectile is not equipped with homing sensors. If the projectile is equipped with homing sensors, then the altitude and orientation in the vertical plane are not necessary for guidance of the projectile towards the target once the target is identified by the homing device. However, the roll angle is of particular importance since it has to be known for proper operation of guidance and control system, i.e., for proper operation of the control surfaces of the projectile such as fins, canards, or other available control surfaces and/or thrusters used for guiding the projectile towards the target indicated by the homing sensors. In addition, gyros or accelerometers employed by inertia devices to measure roll angle (rate of angular rotation or angular acceleration in roll) still need to be initialized following launch and settling of the inertia device and sometimes later during the flight, depending on the flight time and the drift rate of the inertia devices.
A need therefore exists for methods and apparatus for sensors that can be used onboard a flying object such as gun-fired munitions, mortar or rocket to roll the object to a desired roll angle relative to the vertical plane or any other plane of reference. The sensors can provide a real-time measure of the deviation of the object in roll from the desired roll angle, so that the measured deviation can be used by the control system onboard the object to roll it to the said desired roll angle.
The aforementioned roll position indication sensors can be autonomous, capable of being mounted or embedded into various moving platforms, in particular, in various gun-fired munitions and mortars and rockets. The roll position indication sensors can be low cost, capable of being used in guided direct- and indirect-fire munitions, and be small enough to be reliably integrated into small- and medium-caliber munitions as well as long-range munitions.
The disclosed sensors provide a real-time measure of the deviation of the object in roll from the desired roll angle positioning so that the measured deviation can be used by the control system onboard the object to roll it to the desired roll angle. The measured deviation is the difference between the amplitude of the signal received at two symmetrically positioned sensors about a plane of polarization which is used to indicate the desired roll angle positioning of the object. One advantage of this method is that the magnitude of the signal received at each individual sensor does not have to be correlated to the object roll angle since when the two sensors are oriented symmetrically with respect to the plane or polarization, i.e., when the object is oriented in the desired roll angle, the two sensors receive the same signal and the difference between their received signal becomes zero (within the acceptable tolerances). In addition, both sensors are similarly affected by environmental and other noise levels, therefore the effects of the noise in the received signal is minimized.
The disclosed sensors cannot however be used to measure roll angle positioning of the object or similarly bring the object to an arbitrary roll angle positioning without similarly rotating the plane of polarization of the reference source. In certain applications, the object in flight operates about a nearly fixed/desired roll angular positioning, such as most UAVs and many guided munitions. In other applications, the projectile may be flying with roll angles that are not actively controlled (such as in spinning projectiles), and therefore to effectively guide the projectile towards the target, the projectile controller needs to know the roll angle positioning of the projectile at all times.
A need therefore exists for roll angle measuring sensors to address the roll angle measurement requirements for the latter applications.